“Metrology is a bit like being a detective”
Testing quantum hardware at the coldest temperatures
It is intellectual challenge that motivates Daniela Zahn. Together with her colleagues, she set up the cryogenic laboratory at the Fraunhofer EMFT, where she tests hardware components for quantum computers. What she particularly enjoys is the challenging characterisation of qubits.
By Veronika Früh
A group of schoolgirls are standing around a thermal imaging camera in a laboratory at the Fraunhofer Institute for Electronic Microsystems and Solid State Technologies (EMFT). As part of Girls'Day, they have assembled small circuits with different resistors, which they are now examining with the thermal imaging camera. “In places where you can see a lot of heat build-up in microchips, there is often something wrong,” explains Daniela Zahn, who is guiding the girls through the various experiments. What Daniela demonstrates here with simple experimental setups also plays a role in her everyday life as a scientist. The 33-year-old is a research associate at the Fraunhofer EMFT, where she is primarily responsible for testing electronic semiconductor elements and quantum hardware components at the lowest temperatures in the cryostat.
At the Fraunhofer EMFT, various hardware components for quantum computers based on superconducting qubits are being manufactured and researched with the aim of making this technology scalable. “The first topic, of course, is the qubits themselves,” explains Daniela. These are fabricated at the institute on wafers, silicon disks of about 20 cm in diameter. In doing so, the institute mainly uses processes and materials that are compatible with established semiconductor fabrication technology. The idea is to produce many qubits in a reproducible way, according to the scientist. “We then test these qubits,” she continues. “There is a whole zoo of measurements that we implement to learn as much as possible about our qubits.” The fact that many time-resolved measurements are used makes the analysis of qubits one of her more demanding tasks. It becomes even more complicated when qubits are not to be characterized individually, but rather several qubits are coupled – as is necessary for application in quantum computing. Daniela and her colleagues are currently preparing for such measurements in the cryo-lab. “In terms of measurement technology, this is definitely by far the most challenging and exciting,“ says the physicist, and the joy of the challenge is clearly written on her face. But what is actually so complex about the task?”It's not difficult to measure any signal,” Daniela explains, ”but it is difficult to measure the right signal. You have to have a very good understanding of the measurement setup to ensure that you actually see what you wanted to measure.”
Qubits are not the only quantum hardware components that Daniela analyzes at extremely low temperatures in the cryostat – where the temperature is a few millikelvins, close to absolute zero. For 3D integration, a method used in chip manufacturing in which several chips are stacked on top of each other, so-called through-silicon vias (TSVs) and bump interconnects are used to connect the individual components of the different levels. A “must-have” for the scalability of quantum chips, as the scientist explains. “We test here whether it works, whether the connections become superconducting, whether the signal gets through well,” she explains. In addition, wires are needed to connect the qubits on the chip to the computer's control systems. The more qubits there are on a chip, the more connections are needed to control and read each of the computing units individually – with an increasing number of qubits, the space required for the increasing number of connections also grows steadily. To counteract this, the Fraunhofer EMFT is developing a new type of connection: “We are working on flexible cables here, so that you can replace this whole bunch of wires with flexible, thin, superconducting wires,” explains Daniela. “We are also testing these flex cables in the cryostat.” In order to make the most efficient use of the measuring time at the cryostat, she and her colleagues in the cryo-laboratory always try to install as many components as possible in the cryostat at the same time for characterization.
Daniela Zahn, 33
Position
Research associate
Institute
Fraunhofer Institute for Electronic Microsystems and Solid State Technologies (EMFT)
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Degree
Physics
Daniela tests various hardware components for quantum computers. In the cryo lab at Fraunhofer EMFT, she analyses qubits, so-called through-silicon vias (TSVs) and flexible superconducting connections, among other things, at extremely low temperatures. Her analysis of quantum computing hardware is an important step on the way to scalable quantum computers based on superconducting qubits.
Optimal mix of research and application
What Daniela particularly likes about her work at the Fraunhofer EMFT is that she is able to conduct application-oriented research. She began her scientific career in basic research. After completing her bachelor's and master's degrees in physics at the Freie Universität Berlin, she did her doctorate at the Fritz Haber Institute of the Max Planck Society in Berlin. “I was also in the field of solid-state physics, but much more in basic research,” the scientist explains. At the microscopic level, she examined how electrons in solids interact with the crystal lattice and with the magnetic order, which are the basis of many properties of solids. “What I always found a bit tedious about basic research is that you don't yet see the result or the product,” she recalls. You never know whether the work will ultimately lead to an application. At the Fraunhofer EMFT, she has “an optimal mix” of research and proximity to application, which motivates her greatly. “Particularly in the case of application-oriented research, you are potentially doing something that will advance technology,” she says, pleased.
For two years now, Daniela has been working in the cryogenic laboratory of the Fraunhofer Institute, in which she played a major role in setting up. From her time as a doctoral student, she already had some knowledge of vacuum and cryogenic technology, but the electrical measurements of the qubits were new to her. “Before that, I did more optics,” Daniela explains, “but ultimately, these are all waves. Some are in the optical range, the others are microwaves.” Many principles are similar, so it was not difficult to get into the new topic. She also describes her approach to quantum mechanics in a similarly pragmatic way: ”Nature behaves as nature behaves. You don't have any choices, you just have to accept it. Then you have a theory that describes it and you live with it.” So she just lives with it and takes care of the measurements of “everything that is produced here in the house”.
Supporting young scientists
In addition to the actual laboratory work – “getting measurements to work or writing Python code” – Daniela has also spent a lot of time setting up the cryo-laboratory over the last two years: “In the beginning, I did a lot of room planning. Where do you put what, what equipment do I buy, is it the right equipment for my measurements?” It was a lot of work, but it paid off: she and her colleagues received the institute's internal ‘Team Award’ for the quick and successful construction of the laboratory. And even now, when she is standing in her cryo-laboratory, the scientist is already thinking about the next construction work: “We are getting a second cryostat. So I ask myself, what could be improved, what could we need in the future. And I look forward to all the possibilities and cool measurements we still want to do.”
She is particularly motivated by the wide range of complex tasks involved: “I definitely seek intellectual challenges,” says the scientist. “And when it comes to measurement technology, it's a bit like being a detective. You get a few clues and then you have to understand what's going on. I just love that.” The fact that physics has a lot to do with understanding is what convinced her to study the subject in the first place. After graduating from high school, Daniela had initially started studying medicine, but quickly quit – too much memorization for her taste.Now she feels she is doing something that is more in line with her strengths and is also useful. Or something that at least has the chance of being useful, as she puts it – after all, in research you never know what the end result will be.
“What also always motivates me a lot is working with younger people and helping them on their own scientific journey,” she adds. Her most important advice to all young scientists is not to be intimidated – not even by physics, for example – and to do what you basically enjoy, what you are best at. In retrospect, she herself finds it a bit of a shame that she never saw herself in engineering professions. “I just never thought of it as an option,” she explains. Even as a woman studying physics, she was somewhat out of line with convention; if she had studied electrical engineering, the gender imbalance would have been even more pronounced. During her studies, however, it wasn't something she thought about much, rather beforehand and afterwards. “I was the only female student in the advanced physics course. I sometimes felt like an alien, at 17 or 18 years old,” she says. Even during her studies, she was one of few women, but it didn't bother her as much anymore and she didn't see it as a disadvantage. After all, it was relatively clear what you had to do to be successful. “Study, pass your exams, and you'll be fine. I always wondered why such a big fuss was made about such topics,” Daniela summarizes. ”Once you've finished your studies, it's less clear what the success factors are. It's more a matter of wondering how to stand your ground, how to be taken seriously, things like that, small nuances.“ Today, the scientist herself is in a position to provide dedicated support for young researchers.”But I wouldn't limit it to girls or women,” she emphasizes. ”I just think it's important to provide appropriate support to young people who are motivated and full of energy.” That's why she always tries to offer interested parties an internship at the Fraunhofer EMFT and attaches great importance to providing students with the best possible support and supervision for their theses. “Of course I would hope that more girls would dare to choose physics if they were interested,” she says with a smile. The group of schoolgirls from Girls'Day can now at least already imagine what that might look like.
Published 28 February 2025; Interview 26 November 2024